Discussion Forum

Nico,

there are several issues.

1. Your geometry is kilometers big. You may have chosen the wrong geometry unit.

2. The port must connect the stripline and ground unless I misunderstand your design.

There is a microstripline example in the application library you can use as a guide for a proper setup.

Cheers Edgar

1. Thank you for pointing out the scaling problem. I unfortunately overlooked it while looking for issues. An issue still remains as now the S21 parameter is in the -30dB which is in the same order of magnitude as with other similar designs and it doesn't seem like propagation is happening along the conductor.

2. I'm not trying to use microstriplines but cpw lines. I have tried to use the cpw example for modeling my designs but have been unable to find success. Regular ports nor lumped ports seem to work for my specific example as there is no propagation along the conductor.

I'm thinking it might have something to do with how I am assigning ports on interior boundaries but I am having a hard time pinpointing the nature of the problem.

I have an example photo of the electric field where I used rectangular ports. This kind of phenomena of the electric field being strongest at the back of the port seems to be happening with all types of ports, lumped ports included.

Regards, Nico

I still think, that the lumped ports need to connect the conductor and ground.

Thank you, seems to have worked in solving this issue as there is now propagation between ports, although this seems to only happen with uniform lumped ports. The problem still persists with other types of lumped ports and regular ports.

On another note what do you think would be the most accurate port type for modeling these sort of resonators? I mean these types where ports are on an inner surface. And I would like you do add what should be taken into account when modeling using your suggested port type.

When working with RF transmission lines of any kind, using a lumped port means making a circuit approximation to the true EM field configuration at the subject port. Often, especially if: (1) you choose a lumped port with close to the correct impedance, and (2) its dimensions are very small compared to the wavelength in the medium in question, this may (i.e., still not always, but often) be a very good and adequate approximation for many/most purposes. However, if you want or need something more accurate, then you need to specify (or solve for) the detailed field distribution at the port(s). A user-defined or numerical port can potentially provide a more accurate representation, but the price is that you will need to put in more work (and more study & thinking) to properly specify or compute (whether analytically or numerically) the fields at those kinds of ports. Comsol Multiphysics provides a variety of RF port specifications and types. I encourage you to familiarize yourself with many of them, along with studying example applications (see examples in the Application Library) that make use of them.

Not sure what your application exactly looks like. Lumped ports are typically used in a situation when some cable connects the structure. General ports may be more appropriate in case the structure is coupled by radiation.

Edit: This time Robert was faster and his response has more detail.

Thank you, your answers were enlightening and helped me deal with the problems concerning ports and lumped ports, and I've managed to get reasonable results from the simulations.

Regards, Nico